Polyethylene terephthalate (PET) is widely used for the production of light weight plastic articles since PET is excellent in mechanical properties such as formability and creep resistance and can be biaxially molecularly oriented. However, during molding or extrusion processes, acetaldehyde is formed by thermal decomposition of the polyester and when the polyester is formed into an article, the acetaldehyde in the article walls migrates into the contents of the article. Small amounts of acetaldehyde adversely affect the flavor retaining property of foods and beverages, and the fragrance retaining property of foods, beverages, cosmetics, and other package contents. For these reasons, it is desirable to minimize the migration of acetaldehyde into package contents.
Thermoplastic polyesters excellent in gas barrier properties have been proposed. For example, U.S. Pat. No. 4,398,017 discloses a copolyester containing terephthalic acid and isophthalic acid as the acid component, and ethylene glycol and bis(2-hydroxyethoxy)benzene as the diol component. However, when such gas barrier polyester is used as a constituent material of a container, permeation of gases such as oxygen and carbon dioxide are controlled, but acetaldehyde migration into the food or beverage is not controlled, and thus, the flavor and fragrance of the content is effected.
The use of polyamides to increase the gas barrier properties in polyethylene terephthalate resins is disclosed in U.S. Pat. Nos. 4,837,115, 4,052,481 and 4,501,781.
U.S. Pat. No. 4,837,115 discloses a thermoplastic composition containing polyethylene terephthalate and high molecular weight polyamides which act to reduce the residual acetaldehyde contained in the polyester. U.S. Pat. No. 4,837,115 states that the molecular weight of the polyamide is not critical so far as the polyamide has a film forming property. Such polyamides, therefore, must have high enough molecular weights to form a film. It is well known in the art that polyamides having molecular weights of at least 12,000 are necessary to form a film
U.S. Pat. No. 4,052,481, discloses a resin composition containing an aromatic copolyester, a polyamide and a polyalkylene phenylene ester or a polyalkylene phenylene ester ether. The aromatic copolyester contains terephthalic acid, isophthalic acid and a bisphenol.
U.S. Pat. No. 4,501,781, European equivalents, 0212339 and 0092979, disclose a mixture containing 70 to 95 weight percent of a polyethylene terephthalate resin and 5 to 30 weight percent of a xylylene group-containing polyamide resin. U.S. Pat. No. 4,501,781 does not mention acetaldehyde, however, it does state that in order to mold a container that has high gas barrier properties, the resin mixture material should have as close to 30% by weight of the xylylene group containing polyamide resin to PET as possible. Furthermore, it states that the use of 5 to 10% by weight of the xylylene group-containing polyamide resin to PET will result in a container that does not have high gas barrier properties.
The above mentioned patents are deficient because high molecular weight polyamides do not provide adequate reduction in residual acetaldehyde without imparting haze to the polyester. If small amounts of the high molecular weight polyamides are used in such patents, an acceptable level of haze can be achieved, however, residual acetaldehyde is very large. On the other hand, if larger amounts of the high molecular weight polyamides are used, residual acetaldehyde can be reduced but only at the expense of haze.
In contrast, the present inventor has unexpectedly discovered that polyamides selected from the group consisting of low molecular weight partially aromatic polyamides having a number average molecular weight of less than 15,000 and low molecular weight aliphatic polyamides having a number average molecular weight of less than 7,000, used in a critical amount are more effective in reducing residual acetaldehyde in polyethylene terephthalate based polyesters than high molecular weight polyamides. Moreover, the low molecular weight polyamides of the present invention used in a critical amount of 0.05 to 2.0 weight percent of the PET do not produce haze. Thus, the present invention has overcome the trade off wherein low residual acetaldehyde could only be achieved at the expense of haze.